Add a proper check for yarn

Before, `isYarn` would be false if you ran yarn using `npx`.

Author: MarmadileManteater

Diff for: lib/run-task.js

@@ -157,7 +157,7 @@ module.exports = function runTask(task, options) {
         const npmPath = options.npmPath || process.env.npm_execpath //eslint-disable-line no-process-env
         const npmPathIsJs = typeof npmPath === "string" && /\.m?js/.test(path.extname(npmPath))
         const execPath = (npmPathIsJs ? process.execPath : npmPath || "npm")
-        const isYarn = path.basename(npmPath || "npm").startsWith("yarn")
+        const isYarn = process.env.npm_config_user_agent && process.env.npm_config_user_agent.startsWith("yarn") //eslint-disable-line no-process-env
         const spawnArgs = ["run"]
 
         if (npmPathIsJs) {